The family moved to Venezuela, then Denmark, and eventually settled in Argentina in the provinces of Córdoba, Chaco, and Buenos Aires where he attended primary school and acquired a fluent proficiency in Spanish while his father managed poultry farms and ranches.[7] At the age of 11, his parents divorced and he returned to New York with his mother where he spent the rest of his childhood, graduating from Watertown High School in Watertown, New York.[8]

After several years of experience in live theater, Mortensen made his first film appearance playing an Amish farmer in Peter Weir's Witness (Mortensen had actually acted in at least one prior film, The Purple Rose of Cairo, but his scenes were deleted from the final cuts). Also in 1985, he was cast in the role of Bragg on Search for Tomorrow. Mortensen's 1987 performance in Bent at the Coast Playhouse, Los Angeles, won him a Dramalogue Critics' Award. Coincidentally, the play, about homosexual concentration camp prisoners, was originally brought to prominence by Ian McKellen, with whom Mortensen later costarred in The Lord of the Rings. In 1987, Mortensen guest starred as a corrupt police detective on the hit series Miami Vice.[11]

Another major mainstream breakthrough came in 1999, when Peter Jackson cast him as Aragorn in The Lord of the Rings film trilogy. According to the Special Extended Edition DVD of The Lord of the Rings: The Fellowship of the Ring, Mortensen was a last-minute replacement for Stuart Townsend, and would not have taken the part of Aragorn had it not been for his son's enthusiasm for the J. R. R. Tolkien novel. In the The Two Towers DVD extras, the film's swordmaster, Bob Anderson, described Mortensen as "the best swordsman I've ever trained." Mortensen often spent days hiking to the film's remote locations, in costume and carrying his sword, in order to appear authentically travel-worn; he also performed all of his own stunts, and even the injuries he sustained during several of them did not dampen his enthusiasm. At one point during shooting of The Two Towers, Mortensen, Orlando Bloom, and Brett Beattie (scale double for John Rhys-Davies) all had painful injuries, and during a shoot of them, running in the mountains, Peter Jackson jokingly referred to the three as "the walking wounded." Also, according to the Special Extended Edition DVD of The Lord of the Rings: The Return of the King, Mortensen purchased the two horses, Uraeus and Kenny, whom he rode and bonded with over the duration of the films.

In 2004, Mortensen starred as Frank Hopkins in Hidalgo, the story of an ex-army courier who travels to Arabia to compete with his horse, Hidalgo, in a dangerous desert race for a contest prize.

In 2005, Mortensen starred in David Cronenberg's A History of Violence. He was nominated for a Satellite Award for Outstanding Actor in a Motion Picture for this role. In the DVD extras for A History of Violence, Cronenberg related that Mortensen is the only actor he had come across who would come back from weekends with his family with items he had bought to use as props on the set.

In September 2007, the film Eastern Promises, directed by David Cronenberg, was released to critical acclaim for the film itself and for Mortensen's performance as a Russian gangster on the rise in London. His nude fight scene in a steam room was applauded by Roger Ebert: "Years from now, it will be referred to as a benchmark."[13] Mortensen's performance in Eastern Promises resulted in his winning the Best Performance by an Actor in a British Independent Film award from the British Independent Film Awards.[14] He was also nominated for an Academy Award for Best Actor.[15]

While it was reported in April 2009 that Mortensen had, at least temporarily, retired from film acting,[18] Mortensen said he was misquoted. In a 2012 interview, he denied that he ever said he was retiring, only that he didn't have "plans to do another movie" at the time and that he was "taking a little break now. I don’t have anything lined up.”[19]

With part of his earnings from The Lord of the Rings, Mortensen founded the Perceval Press publishing house — named for the knight from the legend of King Arthur — to help other artists by publishing works that might not find a home in more traditional publishing venues.[23]

Perceval Press is also the home of Viggo's many personal artistic projects in the area of fine arts, photography, poetry, song, and literature (see below).

Mortensen is also an author, with various books of poetry, photography, and painting published. His bibliography includes:

Ten Last Night – (1993), his first collection of poetry.

Recent Forgeries – (1998), ISBN, 5th Edition, documents Viggo's first solo exhibition and includes a CD with music and spoken-word poetry. Introduction by Dennis Hopper.

Errant Vine – (2000), limited edition booklet of an exhibit at the Robert Mann Gallery.

Hole in the Sun – (2002, ISBN), color and black & white photographs of a back yard swimming pool.

SignLanguage – (2002 ISBN), a catalog from an exhibition of his works, combining photographs, paintings, and poetry into a multimedia diary of his time in New Zealand while filming The Lord of the Rings: The Fellowship of the Ring. Introduction by Kevin Power.

Coincidence of Memory – (2002, ISBN) Third Edition. In this book, the artist combines photographs, paintings, and poems that cover his artistic output from 1978 to 2002.

45301 – (2003), ISBN. Abstract images, fragments, and phrases from poems comprise this photography book. Many of the photographs were shot during travels to Morocco, Cuba, and the northern plains of the United States.

Un hueco en el sol – (2003), a small booklet published to accompany the exhibition "Un hueco en el sol" at the Fototeca de Cuba in Havana. In Spanish.

Miyelo – (2003), a series of panoramic photographs of a LakotaGhost Dance. It also tells about the events leading up to the massacre at Wounded Knee.

The Horse is Good – (2004), ISBN, a photography book, partly shot during his work on the film Hidalgo, about horses as partners, teachers, and fellow travelers. Images from Morocco, South Dakota, Montana, California, Iceland, New Zealand, Denmark, Brazil, and Argentina. This book reflects Mortensen's fondness for horses. In fact, he bought Uraeus—the horse who played Brego, Aragorn's steed (Roheryn in the books) in The Lord of the Rings movies—as well as TJ, one of the horses who played Hidalgo. He also purchased the stallion that played Arwen's horse, a grey Andalusian stallion named Florian, and gave it to the stunt woman, Jane Abbott, who rode the horse in place of Liv Tyler.

Linger – (2005). In this book, the artist combines black and white photographs and prose poems. Images from Spain (partly shot during his work on the film Alatriste), Morocco, Iceland, United States, Denmark...

Sådanset – (2008). A small booklet published to accompany the exhibition Sådanset (October 18 – November 16, 2008) at the Palæfløjen in Roskilde (Denmark).

Canciones de Invierno – Winter Songs – (2010). Collection of photographs and poems. Bilingual : in Spanish and English. It includes new texts (most of the poems and all the translations) and revised versions of texts previously published. They're accompanied by pictures of landscapes taken during the previous two winters.

Mortensen is a painter and photographer. His paintings are frequently abstract and often contain fragments of his poetry in them. His paintings have been featured in galleries worldwide, and many of the paintings of the artist he portrayed in A Perfect Murder are his own.[24]

Mortensen experiments with his poetry and music by mixing the two art forms. He has collaborated with guitarist Buckethead on several albums, mostly released on his own label (Perceval Press) or TDRS Music. Viggo was first introduced to Buckethead's work while working on sounds for an educational CD on Greek mythology. The finished product included a guitar part by Buckethead, which caught Viggo's ear and led him to initiate contact with the guitarist. The collaboration grew from there.[25]

Mortensen holds dual American-Danish citizenship.[28] He speaks fluent English, Danish, and Spanish; he is also conversational in French and Italian, and understands Norwegian and Swedish. He has stated that he was raised speaking English and Spanish and at times feels more comfortable expressing himself in Spanish.[29] He also has some knowledge of Catalan; twice, when receiving a prize in Catalonia, he made a short speech in Catalan.[30]

Mortensen met his wife, actress/singer Exene Cervenka in 1986 on the set of the comedy Salvation!, a parody of televangelism. The couple married on July 8, 1987. On January 28, 1988, Cervenka gave birth to a son, Henry Blake Mortensen. Mortensen and Cervenka lived in Idaho for three years.[31] Mortensen and Cervenka separated in 1992, and were divorced in 1997.[32] He has been with partner Ariadna Gil, a Spanish actress, since 2009.

Mortensen was a good friend of Icelandic painter Georg Guðni Hauksson up until his death in 2011. Mortensen had long been an admirer of Hauksson's work as a landscape artist, and the two published books together as well as maintaining a close friendship.[37][38]

^Stone, Andrew; Carolyn Bain; Michael Booth; Fran Parnell (2008). "Cinema". Lonely Planet Denmark. Lonely Planet. p. 46. ISBN978-1-741-04669-4. Viggo Mortensen catapulted to fame as Aragorn in the Lord of the Rings trilogy. Although he was born in New York and has lived outside of Denmark for most of his life, he retains Danish citizenship.